Four missiles targeting a vehicle killed four militants in the village of Boya in Miran Shah, a part of the mountainous North Waziristan tribal region that borders Afghanistan.

Earlier, two other drone strikes -- one in Miran Shah and the other in Mandi Khel -- killed 13 suspected militants, said the two Pakistani intelligence officials who asked not to be named because they are not authorized to speak to the media. The strikes were aimed at suspected militants' hideouts.

The three unmanned aircraft missile attacks started 2011 off with a high count. There were an estimated 111 U.S. drone strikes in 2010, and 52 in 2009.

Intense drone activity in Pakistan's tribal region has moved northward, mirroring the movement of suspected militants as they try to flee the targeted strikes, a senior Pakistani military official told CNN.

The United States does not officially confirm that it has unmanned aircraft firing missiles at terror targets in Pakistan, but it is the only country in the region known to have the capability to do so.